Silicon-on-Insulator (SOI) optical devices may include an active surface layer that includes waveguides, optical modulators, detectors, CMOS circuitry, metal leads for interfacing with external semiconductor chips, and the like. Transmitting optical signals from and to this active surface layer introduces many challenges. For example, a fiber optic cable may be attached to the SOI optical device and interface with a waveguide on its surface layer. However, the mode field diameter of the one or more modes of the optic cable (e.g., approximately 10 microns for a single-mode cable) may have a much different size than the mode of a sub-micron dimensioned waveguide tasked with routing the optical signal in the SOI device. Accordingly, directly interfacing the fiber optic cable with the sub-micron waveguide may result in low transmission efficiency or high coupling loss (e.g., less than 1% coupling efficiency).
In some optical devices, lenses are used to focus the light from an external fiber optic cable or a laser source into the waveguides, thereby shrinking the mode or adjusting the numerical aperture such that the optical signal can be efficiently transferred into the sub-micron waveguides. However, using lenses increases the cost and complexity of the optical device. Moreover, the lenses need to be aligned to ensure the signal from the light-carrying medium focuses onto the waveguide. As such, not only do the lenses add cost to an optical system, but coupling efficiency suffers if the lenses are not aligned correctly.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.